1. Field of the Invention
The present invention relates to ball screws used for transferring/tightening an object in a machine tool or in a molding machine.
2. Background Art
As disclosed in U.S. Pat. No. 1,831,080, a conventional ball screw has a structure in which a screw shaft with threads having an equal pitch is screwed into a nut through a multiplicity of balls such that when any one of the screw shaft and the nut is rotated, the other is moved axially through threads to move or tighten the object.
The problem with this prior art is that the feed speed and torque are at all times constant because the thread pitch is constant. Therefore, so far as the rotational speed does not change, the feed speed and torque cannot be changed in the course of feeding the screw shaft or the nut.
As disclosed in published unexamined Japanese patent application Sho. 63 (1988)-3920, such problem is solved by providing threads different in pitch to each of ends of the screw shaft, and a nut on each of the ends of the screw shaft, wherein when a high-speed low-torque is required, a nut for a smaller pitch is turned to feed the screw shaft out while when a low-speed high-torque is required, the screw shaft is rotated to feed out a nut for a larger pitch.
However, since the conventional structure requires means for turning one nut, means for rotating the screw shaft and means for switching between the rotation of the screw shaft and that of the nut, the overall structure is complicated and much time is required for operating such ball screw.